1. Field of the Invention
The present invention relates to a heat treatment apparatus for heating or cooling a substrate such as a semiconductor, a liquid crystal panel or a photomask thereof.
2. Description of the Related Art
A conventional heat treatment apparatus will first be described, taking up the case of manufacturing a photomask of a semiconductor device, for example, an LSI.
A manufacturing process of a photomask of an LSI now in use generally comprises a step of sputtering a chromium film having a thickness of about 0.1 xcexcm onto a glass substrate having a main surface of a square 152 mm on a side and a thickness of 6.3 mm, a step of coating the substrate with a resist and writing a pattern with electron beams, a step of developing the resist, a step of etching the chromium film, and a step of stripping off the resist.
Along with the recent improvement of integrity and refinement of patterns in the LSI, a chemical amplification type resist providing a high dry etching resistance and giving a high-accuracy resist pattern was developed and industrialization is now in progress. A typical commercial product is SAL601 made by Shipprey Co.
The sensitivity amplification of the chemical amplification type resist is conducted through baking at about 110xc2x0 C. after a pattern is written with electron beam. This baking is called as PEB (post exposure bake). On this count, a size of the resist pattern after development largely varies as a result of variation of temperature upon baking. In order to reduce variation of the size of the resist pattern, it is required to conduct a heat treatment so as to achieve a uniform temperature in the photomask.
As an apparatus or a method for uniformly baking a substrate, some proposals are disclosed in Japanese Unexamined Patent Application Publications JP-7-5447, JP-7-211628, and JP-7-281453.
Japanese Unexamined Patent Application Publication JP-7-5447 discloses a substrate heating apparatus comprising a heater, a hot plate holding the heater, and a lower case, wherein a liquid is sealed in a space between the heater and the hot plate/the lower case.
Japanese Unexamined Patent Application Publication JP-7-211628 discloses a substrate heating apparatus comprising heating means, a hot plate having a concentric circular recess formed therein, temperature detecting means and temperature adjusting means.
Japanese Unexamined Patent Application Publication JP-7-281453 discloses a substrate heating method comprising the steps of placing a substrate to be heated on a bottom hot plate having a heater, and making close the lower surface of a top hot plate having a heater to the outer periphery of the substrate to be heated so as to heat the substrate from the bottom and top surfaces thereof.
All these proposals have an object to achieve a uniform temperature in the substrate during a steady state at a high temperature of, for example, 110xc2x0 C.
However, along with improvement of integrity of semiconductor integrated circuits, it has gradually been known that, not only temperature uniformity during a steady state at a high temperature, but also temperature uniformity during a transition state in heating and cooling has an important effect on improvement of accuracy of the pattern size.
In the conventional heat treatment apparatus, the hot plate is larger in size as compared with the substrate. When a thick substrate such as a photomask for semiconductor is mounted and heated on a conventional heat treatment apparatus, heat is supplied also from the portion of the hot plate protruding from the substrate. As a result, a problem is encountered in that temperature of the outer portion of the substrate rises rapidly in transition. Similarly, when cooling a heated substrate, a problem is encountered in that temperature of the outer portion falls rapidly in transition.
Using a conventional heat treatment apparatus comprising a bottom hot plate and a top hot plate, a 152 mmxc3x97152 mm square photomask was heated and cooled, and changes in temperature were measured at nine points positioned at intervals of 65 mm in this photomask. The result of measurement is shown in FIGS. 1A and 1B.
FIG. 1A illustrates a time change in average temperature on the nine points in the photomask. In FIG. 1A, the line indicated by reference numeral 21 represents the ramp up characteristics in heating the photomask; the line indicated by reference numeral 22 represents characteristics during a steady state at a high temperature; and the line indicated by reference numeral 23 represents the ramp down characteristics in cooling the photomask.
FIG. 1B shows a time change of the maximum temperature difference among the nine points in the photomask. Because temperature of the outer peripheral rises rapidly in heating, the temperature difference was measured to be 5.5xc2x0 C. The difference becomes smaller as 0.5xc2x0 C. during the steady state at the high temperature, but as the temperature of the outer periphery falls rapidly in cooling, the temperature difference was measured to be 8.3xc2x0 C. When there are large temperature differences among a plurality of points in transition such as in heating and cooling, a problem is caused of the poor size accuracy of patterns.
To solve the aforementioned problems, the present invention has an object to provide a heat treatment apparatus which reduces a temperature difference within a surface of the substrate in transition of heating and cooling and in a steady state, thus permitting manufacture of a resist pattern of a high accuracy.
An apparatus according to a first aspect of the present invention comprises a thermal plate having a main surface containing a first area on which the substrate is to be placed and a second area surrounding the first area, heat capacity per unit area in the second area of the main surface being smaller than heat capacity per unit area in the first area of the main surface; and a temperature control element for controlling temperature of the thermal plate in accordance with supplied current.
In this apparatus, it should be noted that a portion of the substrate may be placed in the second area of the main surface of the thermal plate.
An apparatus according to a second aspect of the present invention comprises a first thermal plate having a main surface on which the substrate is to be placed; a first temperature control element for controlling temperature of the first thermal plate in accordance with supplied current; at least one second thermal plate arranged along an outer periphery of the first thermal plate via a spacer having a heat conductivity lower than that of the first thermal plate; and at least one second temperature control element for controlling temperature of the at least one second thermal plate in accordance with supplied current.
An apparatus according to a third aspect of the invention comprises a first thermal plate having a main surface on which the substrate is to be placed; a first temperature control element for controlling temperature of the first thermal plate in accordance with supplied current; a first temperature sensor for detecting temperature of a predetermined portion of the first thermal plate to output a detection signal; first current control means for controlling the current to be supplied to the first temperature control element in accordance with the detection signal output from the first temperature sensor; a plurality of second thermal plates arranged along an outer periphery of the first thermal plate via a spacer having a heat conductivity lower than that of the first thermal plate; a plurality of second temperature control elements for controlling temperature of the plurality of second thermal plates in accordance with supplied current, respectively; a plurality of second temperature sensors for detecting temperature of predetermined portions of the plurality of second thermal plates to output detection signals, respectively; and second current control means for individually controlling the current to be supplied to the plurality of second temperature control elements on the basis of the detection signals output from the plurality of second temperature sensors, respectively.